1. Field of the Invention
The present invention relates to a method of producing highly abrasion resistant machine elements, and more particularly to a method of producing abrasion resistant plastification elements, such as plastification screws and plastification cylinders for injection molding machines, using temper steel or tool steel which is suitable for surface treatment by boriding, the plastification elements being particularly suitable for use in conjunction with the injection molding of thermoplastic and thermo-setting plastic materials containing mineral fillers, metallic fillers, or other highly abrasive fillers.
2. Description of the Prior Art
It is known in the prior art to increase the surface hardness of plastification elements by subjecting the machined elements to a boriding treatment, packing them in boron carbide powder inside a suitable container which is then placed into a pre-heated compartment furnace and heated at the required boriding temperature, viz. between approximately 800 and 1100.degree. C. Upon termination of this treatment, the containers are removed from the furnace and cooled in still air. After cooling, the plastification elements are removed from the steel sheet container and, to the extent that these elements are plastification cylinders and the boron carbide powder had been preferably filled into the bore of the cylinder, the former is removed from the bore. The necessary tempering treatment, or hardening treatment, as the case may be, is then performed in a separate operation. Heating to the hardening or tempering temperature is performed in an inert salt bath or inside an inert gas furnace. A comparatively mild chilling rate assures that no distortions are created in the borided surface zone so that crack formation is avoided. Experience has shown that this method produces the heaviest and best spiked borided zones, when it is used on non-alloyed and low-alloyed steels. On the other hand, it was found that, when steel with a chromium content in excess of approximately 6 percent is used, the boride zone adjoins the matrix almost smoothly, with the resulting tendency of shearing off under certain stress conditions (Kunst: "Neuere Verfahren zum Erhohen der Lebensdauer von Schnecken zum Verarbeiten von Kunststoffen", in Kunststoffe, Vol. 62, Issue No. 11, 1972).
Using the above-described heat treatment method, it has further been found, for example, that a plastification screw made of steel having the designation 50 CrMo 4, when used in conjunction with the injection molding of thermo-setting plastic material with a mineral filler, had its operative life increased by a factor of 26, as compared to the previously used plastification screw. Other steels which have been found to be suitable for this purpose are steels designated 42 CrMo 4 and 50 CrMo 4 (Kunststoffberater, Vol. 19, Issue No. 1, page 21).
Research concerning the spike formation in the transition zone between the boride zone and the base steel has further revealed that, in contrast to the spike-suppressing, i.e. smoothening tendency of the alloy components molybdenum, tungsten, chromium and vanadium, the alloy component nickel does not exhibit the same undesirable influence on the micro-spike formation in the boride phase, even when present in a concentration of up to 9 percent. This fact has been demonstrated in connection with an air-hardening steel of the designation X 8 Ni 9 - Steel No. 1.5662 (Oberflachentechnik, No. 11/1972, pages 431-436).
From a still further research report is known that borided alloyed steels can be quenched in oil, following removal from the salt bath in which they are heated to the required hardening temperature. According to this source, such treatment does not affect the shape or hardness of the boride zone (Metall, Issue No. 1, Jan. 1973, pages 10-13, esp. p. 12).
Lastly, it is also known that the presence of certain alloy components in the steel, especially the presence of chromium, molybdenum, tungsten, and/or vanadium, as well as carbon, tends to inhibit the growth of the boride zone, in addition to the earlier-mentioned inhibition of the spike formation. It has therefore been recommended to restrict the heat treatment to elements having a relatively thin boride zone (DURFERRIT-Technische Mitteilungen, Durf 21-0-2-573 Bi).